1. Field of the Invention
This invention concerns a surface emitting laser device having a vertical cavity, and an optical module and an optical system using the same.
2. Statement of the Related Art
Along with explosive increase of Internet users in recent years, high speed information transmission has been required in local area networks (LAN). It is expected that transmission rate at Gb/s level for end users and in excess of 10 Gb/s level for backbones connecting between HUBs will be required in the next 5 to 10 years. Therefore, it is considered that optical communications using optical fibers as far as end users are necessary in the near future. Usually, for optical communication, semiconductor lasers, photo-detectors and optical modules incorporating driving circuits therein are used. In optical modules to be used in future LAN, it is indispensable that these optical modules are provided at a reduced cost for use by a great number of users. In addition, high speed transmission performance in excess of 10 Gb/s must be possible.
FIG. 1 shows a schematic view for a high speed optical module in excess of 10 Gb/s known so far.
As shown in FIG. 1, the optical module comprises a semiconductor laser device 401, a laser driving circuit 402, an external modulator 403, a TEC (Thermo Electric Cooler) for stabilizing the temperature for the device 404, a photodetector 405, a photodetector driving circuit 406, an entire optical module package 407, an external circuit 408 for operating the optical module, and an optical fiber 409. The optical module generates a laser beam from the semiconductor laser device 401 in accordance with the external circuit 408. The high-speed modulated light in excess of 10 Gb/s is transmitted through the external modulator 403. Further, optical signals transmitted from a mating optical module is received by the photodetector 405. All the optical signals are transmitted and received through the optical fiber 409. As the semiconductor laser device 401, an edge emitting laser using gallium indium phospho arsenide (GaInPAs) series semiconductor material for the active layer is mainly used. The laser beam wavelength is at 1.3 xcexcm or 1.55 xcexcm applicable to a single mode fiber capable of long distance and high speed transmission.
Generally, the GaInPAs series laser has a drawback that a threshold current increases remarkably when a device temperature increases. Accordingly, it has been necessary to incorporate a temperature stabilizing thermoelectric cooler. As described above, the number of parts constituting the optical module is large and, therefore, the size of the module is large and the cost of the optical module itself is expensive. This is a concern in that the existent level of transmission rate of 10 Gb/s has been mainly used in trunk transmission networks in which performance rather than the cost is emphasized. In view of the above, existent 10 Gb/s optical module is not essentially suitable for the application to future LANs which require cost reduction. The dotted lines in FIG. 1 denote partitioning between the light transmission side in which a semiconductor laser device is disposed and a light receiving side in which the photodetector is disposed, and each of the portions may serve as an optical transmission module and an optical receiving module independently. Further, the photodetector for optical output monitor of semiconductor laser device is omitted.
Recently, a surface emitting laser has attracted attention as a light source suitable for use as a high speed optical module for future LANs. The surface emitting laser has a cavity length as small as several xcexcm which is much shorter when compared with the cavity length (several hundreds xcexcm) of the edge emitting laser and is basically excellent in high speed characteristics. Further, the surface emitting laser also has excellent features in that (1) the beam shape is nearly circular which is easily coupled with an optical fiber (2) a cleaving step is not necessary in the production step and device check is possible on the wafer and (3) the laser oscillation is conducted at a low threshold current and consumes less electric power to reduce the cost. As for the lasing wavelength, the lasing operation at 1.3 xcexcm range by using new semiconductor materials which can be formed on a gallium arsenide (GaAs) substrate such as of gallium indium nitrogen arsenide (GaInNAs) or gallium arsenic antimonate (GaAsSb) have been reported successively in recent years.
For the semiconductor laser devices, it has been expected that more and more practical surface emitting lasers in a long wavelength range are adaptable to a single mode fiber for long distance and high speed transmission. Particularly, it is expected that when GaInNAs is used for the active layer, electrons can be confined in a deep potential well in the conduction band and the stability of the temperature characteristics can also be improved drastically. It has been expected for the long wave range surface emitting laser device using GaInNAs as the active layer that it can provide an optical module of high performance, at a reduced cost and suitable to use in LANs based on the foregoing advantages.
The surface emitting laser basically comprises an active layer for generating light, a current confinement layer for injecting current to a minute region of the active layer and an optical cavity comprising a pair of reflectors disposed so as to vertically put the active layer therebetween. Usually a semiconductor Distributed Bragg Reflector (DBR) is used as a reflector and the current is injected by way of a semiconductor DBR layer into the active layer.
Since the semiconductor distributed Bragg reflector (DBR) has high resistance, a surface emitting laser of a different structure in which current is injected not by way of the reflector has also been studied. An example is a surface emitting laser as described in Japanese Patent Laid-Open Hei 11-204875 (laid-open on Jul. 30, 1999). FIG. 2 shows a device structural view of a surface emitting laser. As shown in FIG. 2, the surface emitting laser comprises a lower electrode 501, a semiconductor substrate 502, a lower DBR 503, a first spacer layer 504, an active layer 505, a 20 second spacer layer 506, a current confinement layer 507, a current induced layer 508, a third spacer layer 509, an upper electrode 510 and an upper DBR 511. Since the upper electrode 510 is disposed on the side of the upper DBR 511, the induced current from above is introduced from the third spacer layer 509 through the current induced layer 508 to the aperture restricted by the current confinement layer 507 and then introduced into the active layer 505. That is, since the current is induced not by way of the upper DBR 511, the device resistance can be reduced. Further, in this structure, the current induced layer 508 with an increased doping concentration is adopted intending to reduce the resistance to the horizontal direction relative to the substrate between the electrode and the aperture (hereinafter referred to as a resistance to the lateral direction).
This invention intends to provide a surface emitting semiconductor laser device capable of high speed operation. High speed operation, for example, above 10 Gb/s is attained in accordance with this invention.
This invention further intends to provide a surface emitting semiconductor laser device capable of high speed operation and reduction in cost.
This invention further provides an optical module incorporating the surface emitting semiconductor laser device capable of higher speed operation.
For coping with such technical subjects, it is necessary to overcome the foregoing problems in the surface emitting laser. At first, a surface emitting laser device structure capable of injecting current to an active region not by way of an upper DBR of high resistance should be adopted. For this purpose, it is necessary to provide a new method capable of reducing the resistance in the lateral direction of the current passing from the electrode through the aperture and injected into the active region and attain a drastic reduction of the device resistance to about 10 xcexa9.
A typical embodiment of this invention resides in a surface emitting laser device comprising, on a semiconductor substrate, an active region for generating light, a current confinement region disposed on the side opposite to the semiconductor substrate while putting the active region therebetween, an optical cavity comprising reflectors putting the active region and current confinement region vertically therebetween in the layering direction of the semiconductor layer, a first electrode disposed on the side of the semiconductor substrate and a second electrode disposed on the side opposite to the semiconductor substrate while putting the current confinement region therebetween, and having a semiconductor region having a layered structure capable of forming 2-dimensional carriers between the current confinement region and the second electrode.
The semiconductor region having the layered structure capable of forming the 2-dimensional carriers is preferably adapted for the purpose of this invention, particularly, the so-called modulation doped structure. That is, the second embodiment of this invention is a surface emitting laser device having, on a semiconductor substrate, an active region for generating light, a current confinement region disposed on the side opposite to the semiconductor substrate relative to the active region, an optical cavity comprising reflectors putting the active region and the current confinement region vertically therebetween in the direction of layering the semiconductor layer, a first electrode disposed on the side of the semiconductor substrate relative to the current confinement region and a second electrode disposed on the side opposite to the semiconductor layer relative to the current confinement region, and having a layered structure capable of forming 2-dimensional carriers between the current confinement region and the second electrode and the semiconductor region having the layered structure capable of forming td the 2-dimensional carriers has at least a first semiconductor layer containing impurities at high concentration and having a wide band gap and a second semiconductor layer containing impurities at a concentration lower than that of the first semiconductor layer or substantially not containing impurities and having a band gap narrower than that of the first semiconductor layer.
The semiconductor region having the layered structure capable of forming the 2-dimensional carriers provide the effect thereof so long as it is present in at least a portion between the current confinement region and the second electrode. In the example to be described later, the semiconductor region having the layered structure capable of forming the 2-dimensional carriers is formed substantially over the entire surface of the substrate surface. More actual embodiment in the practical production is explained but it is important that the layered structure capable of forming the 2-dimensional carriers is present in a current path between the current confinement region and the second electrode. In the other point of view, a region having a layered structure capable of forming the 2-dimensional carriers constitutes a main portion of the current channel. Accordingly, it is not always necessary to form a layered structure capable of forming the 2-dimensional carriers entirely for the surface in parallel with the substrate surface. Further, other semiconductor layer may be further disposed between the layered structure capable of forming the 2-dimensional carriers and the current confinement region. Also in this case, the effect by disposing the layered structure capable of forming the 2-dimensional carriers can also be obtained.
As described above, in the basic constitution of this invention in which a current flowing from the electrode disposed on the side opposite to the substrate relative to the current confinement region has a horizontal component relative to the substrate, the current component in the horizontal direction is conducted mainly by way of the 2-dimensional carrier gas channel. Specifically, it is attained by a surface emitting laser device wherein the 2-dimensional carrier gas channel is formed of a modulation dope structure in which at least one kind of high concentration dope layer comprising a semiconductor of a wide band gap and at least one low concentration dope layer comprising a semiconductor with a narrower band gap than that (the low concentration dope layer may also include the case of not applying doping) are located in at least a portion between the electrode and the current confinement region.
For attaining the main purpose of this invention as described above, reduction of the resistance in the surface emitting laser device due to p-type conduction is intended and a 2-dimensional hole gas mainly as the carriers is used.
The basic constitution of this invention is as has been described above and main preferred embodiments of this invention will be set forth below.
A first embodiment according to this invention is a surface emitting laser device having, on a semiconductor substrate, an active region generating light, a current confinement region disposed on the side opposite to the substrate relative to the active region and an optical cavity comprising reflectors putting the active region and the current confinement region vertically therebetween, in which the current flowing from the electrode disposed on the side opposite to the semiconductor substrate relative to the current confinement region has a horizontal component relative to the surface of the substrate and the current component in the horizontal direction flows mainly by way of the channel for 2-dimensional carrier gas.
The second embodiment of this invention is a surface emitting laser device as defined in the first embodiment, wherein the 2-dimensional carrier gas channel is formed of a modulation dope structure of in which at least one kind of high concentration dope layer comprising a semiconductor of wide band gap and at least one kind of low concentration dope layer comprising a semiconductor with a narrow band gap than that (also including a case of not applying doping) are located in at least a portion of the electrode and the current confinement region.
A third embodiment of this invention is a light emitting laser device as defined in the second embodiment wherein absorption of the laser wavelength beam by the modulation dope structure is less than 1%, and the modulation dope structure is disposed including the inside of the cavity through which the beam travels. Even when the modulation dope structure is incorporated in the optical cavity, it can be driven sufficiently in the same manner as usual surface emitting laser device so long as the absorption of the laser beam in the structure is less than 1%. Such a problem of laser beam absorption can naturally be avoided by disposing the modulation dope structure to the outside of the optical cavity as exemplified below.
A fourth embodiment according to this invention is a surface emitting laser device as defined in Embodiment 2, wherein the modulation dope structure is disposed to the outside of a light propagating cavity.
A fifth embodiment according to this invention is a surface emitting laser device as defined in any one of Embodiment 1 to 4, wherein the high concentration dope layer in the modulation dope structure is p-type and 2-dimensional carrier gas is comprises holes.
A sixth embodiment according to this invention is a surface emitting laser device as defined in any one of Preferred Embodiments 2 to 5, wherein AlGaAs, AlGaInP or layered structure thereof is used in the high concentration dope layer, and GaAs, GaInAs or layered structure thereof is used for the low concentration dope layer. The group III-V compound semiconductor materials set forth here are materials extremely suitable to the practice of this invention. Such materials can easily form high quality layers on a GaAs substrate capable of easily providing excellent characteristic with a view point of the surface emitting laser device.
Prior to explanation for more preferred embodiments, details for basic concept of this invention is to be additionally explained.
Prior to the description for concrete embodiments, details for the basic concept of this invention is additionally explained.
For attaining an optical module having a high speed operation characteristic in excess of 10 Gb/s, it is naturally necessary to attain high speed characteristic in excess of 10 Gb/s in a surface emitting laser used as a light source. For this purpose, it is indispensable to reduce the resistance (R) the capacitance (C) of the surface emitting laser device.
Generally, the basic modulation characteristic of a semiconductor laser device is evaluated by a modulation frequency at which the optical output of the device is lowered by 3 dB (hereinafter simply referred to as f3 dB). Then, f3 dB is represented by using R and C in accordance with the following equation (1)
f3 dB=1/(2xcfx80Rxc2x7C)xe2x80x83xe2x80x83(1) 
The fact described above is explained, for example, in Advanced Optoelectronic Series xe2x80x9cFoundation and Application of Surface Emitting Laserxe2x80x9d, page 184, written by Kenichi Iga and Fumio Koyama, published from Kyoritsu Shuppan.
From the equation (1) above, it is understood that the device resistance should be reduced to about 10 xcexa9 in order to attain several 10 Gb/s for f3 dB of the device. In this case, the capacitance of the surface emitting laser device is assumed to 500 fF as a general value. If the device capacitance can further be reduced, the allowable amount for the device resistance can be increased, for example, 10 xcexa9 or more but it would be appreciated that reduction of the resistance is important also in this case. Further, the device resistance of about 110 is a low value comparable with that of the edge emitting laser and, if it can be attained, laser driving circuits and the like used so far in the edge emitting laser are applicable. In this case, new development cost is not required, which is advantageous in the reduction of the cost for the optical module using the surface emitting laser device according to this invention.
 less than Comparative Discussion with Prior Art greater than 
As has been described previously, a reflector comprising AlAs/GaAs series semiconductor multi-layered film (DBR) has been used mainly in the surface emitting lasers. In the existent device, an electrode is disposed on the DBR comprising a p-type AlAs/GaAs and current is injected through the DBR into the active layer. In this instance, there is a problem that the energy difference at the hetero interface of an AlAs/GaAs series semiconductor provides a large resistive ingredient for holes of heavy effective mass to increase the device resistance. As a countermeasure, it has been attempted, for example, to introduce an AlGaAs semiconductor layer of gradually changed content to the AlAs/GaAs hetero interface, apply p-type doping only on the AlAs side thereby decreasing the resistive ingredient at the hetero interface. However, since the resistance of the p-type AlAs/GaAs series semiconductor DBR is essentially high, it is difficult to attain drastic reduction of the device resistance.
On the other hand, the surface emitting laser described in Japanese Patent Laid-Open Hei 11-204875 (laid-open on Jul. 30, 1999) described previously is to be studied. This example has a structure of injecting current not by way of an upper p-type semiconductor DBR of high resistance.
The resistance to the lateral direction is in proportion with the sheet resistance (Rc). Rc is represented by the following equation (2).
Rc=1/(Nsxc2x7exc2x7xcexc)xe2x80x83xe2x80x83(2) 
wherein Ns represents a sheetxe2x80xa2carrier concentration, e represents an elementary electric charge, xcexc represents a layer mobility and t represents the thickness of the layer. Ns is represented as a product of carrier concentration (p) and the mobility (xcexc) of the layer (Ns=pxcexc).